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In Yamagata Prefecture on Honshu island, Japan, there is a small, isolated body of water by the name of Otori-ike 大鳥池, which lies high in the mountains, 1,000 meters above sea level. Despite its Japanese name ("ike" means "pond"), the limnology of Otori-ike is not that of a pond. It is in fact a lake, created when a landslide blocked off a mountain stream long ago. The lake is 3.2 kilometers (2 miles) around and 68 meters (223 feet) deep at its deepest point. Located within Yamagata's largest virgin forest, Otori-ike is known for the area's stunning natural beauty, and is a haven for hikers.

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The lake is also known for a perplexing natural mystery. Otori-ike is said to be the home of giant fish lurking within its depths; fish known locally as takitaro.

The Takitaro are said to be enormous fish capable of reaching sizes of up to 3 meters (10 feet) long. Locals have long told of seeing these giant fish in Otori-ike, and the creatures are well integrated into the folklore of the area. Takitaro were once claimed to have the ability to bring in storms, and the sight of one was said to mean that a storm was imminent.

The fish were often said to attack small boats, and were blamed for the occasional disappearance of fishermen. One old story tells of a boat that was pulled under the waves by a Takitaro as horrified villagers looked on. Takitaro were also believed to snatch deer and other animals from the lakeshore. There is one account that describes a Takitaro carcass washed up on shore that when cut open revealed the remains of a deer.

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Residents of the area have claimed to even catch Takitaro on occasion and in fact the fish are widely said to be good eating. A modern report of such a catch occurred in 1917, when workers investigating a floodgate managed to capture a fish that measured 150 cm (5 feet) long and weighed 40 kg (88lbs.). The men reportedly ate the fish, and described its meat as being quite good. Other specimens have reportedly been captured throughout the 20th century as well. Several of these captured specimens have been described as being anywhere from 160 cm (5.3 feet) to 2 meters (6.5 feet) long.

Stories abound of fishermen encountering these monstrous fish right up to the modern day, with accounts of mysteriously mangled nets and fishing poles violently yanked or broken by something very large and strong. One report spoke of something that looked like a "moving log" that was witnessed to bowl right through a fishing net. According to the eyewitness, the fish was almost 2 meters long and had what appeared to be a thick layer of fat.

While locals have been aware of these mysterious fish for a long time, perhaps the sighting that single handedly brought the Takitaro into the limelight and to mainstream consciousness in Japan was made by four mountain climbers in 1982. Tomoya Sawa, Kenzo Matsuda, I. Onodera, and Masakazu Sato, were hiking along Otori-ike's nearby Nao Ridge when they saw something in the lake they could not explain. They noticed several huge fish estimated as being 2 meters (6.5 feet) to 3 meters (10 feet) long languidly swimming through the lake's crystal clear water in a counter-clockwise circular arc. The group observed the fish in fascination for some time before the creatures sunk out of sight. The sighting conditions were perfect, with good weather and glassy, smooth water conditions.

This sighting was a sensation all over Japan, and was plastered over most major newspapers. The tale of giant fish dwelling in this picturesque mountain lake fired up the public imagination. Only adding to this fervor was footage captured by a group of TV reporters investigating Otori-ike in October 1983 in the wake of this sighting. The reporters' footage shows three huge shapes swimming under the surface of the water.

In response to the incredible amount of attention this sighting and the subsequent footage generated, a scientific expedition was mounted to the lake in 1985 in the hopes of obtaining evidence of Takitaro. Scientists conducted a thorough search of the lake using sonar equipment, during which they made some peculiar finds. In the deeper parts of the lake, sonar picked up readings at a depth of 30 to 40 meters (98.5 to 131 feet) of what appeared to be fish much larger than any known to inhabit the area. Although the exact type of fish could not be determined, these sonar images seemed to confirm that something very large and mysterious was indeed lurking in the depths.

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This is a diagram showing the size of a Takitaro next to a human for comparison.
Gill nets laid out by the team also brought up some curious findings. The nets captured several fish that measured 70 cm (2.3 feet) long. While this is not a particularly large size when compared to the 2 to 3 meter lengths reported in most Takitaro cases, the mystery deepened when an ichthyologist identified the fish as being Dolly Varden trout (Salvelinus malma malma). This finding is puzzling for two reasons. First of all is that Dolly Varden are represented in Japan by a landlocked subspecies that only inhabit the northern island of Hokkaido. They were not previously known to be in Otori-ike at all. Second, Dolly Varden typically reach sizes of up to 61 cm (24 inches) for migratory individuals and 46cm (18 inches) for non-migratory ones, which would make 70cm (27.5 inches) uncommonly large for a landlocked trout of this variety. Whether or not these super sized Dolly Varden were connected in any way to the giant Takitaro, they nevertheless represent an unusual finding in their own right.

Although these Dolly Varden were indeed very large individuals for their species, no fish known to be present in Otori-ike are known to get as large as what is typically described in Takitaro reports. So what is going on in Otori-ike? What could the Takitaro possibly be?

The capture of exceptionally large Dolly Varden specimens in the lake has led to speculation that some species of fish in the lake could be exhibiting a form of gigantism. The reasoning behind this hypothesis is that isolated populations of fish could develop what is essentially insular, or island, gigantism. Otori-ike is not connected to any solid inflowing or out-flowing waterways, with the main run up being the Ara river, which is dammed. In terms of biogeography, a pond or geographically isolated lake like Otori-ike is considered an island in that it matches the general definition of "island" as an isolated ecosystem surrounded by unlike ecosystems. In the case of insular gigantism, some organisms are found to become much larger in island environments, and the same conditions can apply to lakes as well.

With fish, we can see this trend for instance in nine-spined sticklebacks (Pungitius pungitius). Studies have shown that landlocked populations of this species present in small, isolated ponds display significant increases in growth rate, extended longevity, and larger overall size compared to populations found in larger lake or marine habitats. This increase in size is particularly pronounced in the absence of any natural predators.

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This type of gigantism is hypothesized to be caused by a relaxation of constraints such as predation and interspecific competition in the island environment. This in turn can lead smaller organisms to evolve towards an "optimal" body size in response to this overly simplified ecosystem, thus allowing for an increase in physical size and lifespan in comparison to individuals inhabiting an environment with such constraints in place. However, these increased characteristics fall dramatically once these factors are imposed.

This presents a problem when considering insular gigantism as an explanation for Takitaro. Otori-ike in fact does have a good amount of competitors and predators inhabiting its waters. The cold mountain lake yields a high oxygen content that is very suitable for predatory, cold water salmonoid fishes such as salmon and trout. Otori-ike is home to natural populations of cherry salmon, and it is also heavily populated by introduced species such as brook trout, which were stocked by the tens of thousands in 1898 during the Taisho era, and rainbow trout that have been intermittently stocked in the lake as well. Considering that insular gigantism is much less likely to be seen in the presence of such predatory and competitive pressures, it seems unlikely that any of these fish species would be particularly inclined towards developing gigantism in this particular habitat. Thus, perhaps it is better to look elsewhere for explanations for the large sizes reported with the mysterious Takitaro.

Another possibility is that we are dealing with some type of fish found in Japan that has somehow exceeded its known range, such as the aforementioned Dolly Varden. While this particular type of trout is not known to achieve anywhere near Takitaro proportions, there is another type of fish in Japan that can get large enough to perhaps account for the reports.

The largest known freshwater fish found in Japan is the Sakhalin taimen (Hucho perryi), also known in Japan as the Japanese huchen, Ito, and stringfish, which are found in Russia and the northernmost Japanese island of Hokkaido. These are some of the longest living and largest salmonoids in the world, living up to 40 years and with the world record size being 210cm (7 feet) long and weighing 110 lbs, 4 oz. Interestingly, the size of taimen have led them to become entrenched in folklore and mystery in other parts of Asia. For instance it is said that giant taimen inhabit China's Lake Kanasi, with some reports putting them at sizes of over 3 tons. A Mongolian legend also tells of a huge taimen trapped in river ice that was gradually eaten by starving herders until the ice melted and it swam away.

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Could a monstrous Sakhalin taimen be behind Takitaro reports as well? The main problem we face here is that in Japan these fish are only known to be present in northern Hokkaido, not the island of Honshu where Yamagata and its Otori-ike are. If taimen are indeed in Otori-ike, then they are either remnant, wayward populations that happened to become trapped in the lake when it formed, or they were intentionally placed there by someone. Considering that with the Dolly Varden there has been at least one species found in Otori-ike that should not be there, it is worth thinking about.

The prospect of intentionally introduced species leads us to other possibilities as well. Perhaps we even need to look at other types of exotic fish from places outside of Japan.

One possible culprit is a type of fish known as the snakehead, with two large species in particular, the Northern snakehead (Channa argus) and giant snakehead (Channa micropeltes), being the primary suspects. These fish, which get their name from their reptilian looking head and snakelike patterns on their body, are native to Asia in parts of China, far eastern Russia, and the Korean peninsula. They are long and slender fish, reaching up to 5 feet in length, and possibly more. Snakeheads are popular as a food fish in their native range, and are actively farmed for their meat. In some places, they were intentionally introduced into lakes or rivers as a potential food source, and were in fact introduced to Japan in the early 20th century for this very purpose.

Snakeheads are perhaps best known as a tenacious, persistent invasive species in many parts of the world where they have been introduced. They are known to be adaptable, voracious predators, and able to quickly overrun natural ecosystems. Any snakeheads in Otori-ike would not even necessarily have had to been dumped directly into the lake. These fish possess an unusual adaptation in the form of sacs that function something like lungs. These sacs can either be used to provide oxygen as the fish swims, or even allow it to survive out of water. Indeed, snakeheads are known to actually wiggle and squirm overland to new habitats and can survive out of water for a couple of days as long as they don't dry out. Due to this feature, snakeheads could have found their way up dammed waterways and just "walked" over to Otori-ike.

Snakeheads are present in Japan and their size and odd appearance could perhaps be the source of Takitaro reports. However, it is unclear whether this tropical fish could survive in such cold, high altitude waters no matter how adaptable it is.

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Other fish from outside of Japan could also be behind Takitaro sightings as well. The most commonly proposed are sturgeon or alligator gar, released into the lake for the purposes of eating or sport-fishing. Unlike the land traversing snakeheads, these species would have to have been directly introduced into the lake for them to be present there. It doesn't seem at the outset too far-fetched considering that brook trout and rainbow trout were extensively introduced to the lake as food fish and for sport, and this has also happened on a smaller scale in lakes throughout Japan with unscrupulous anglers secretly releasing black bass.

Some exotic species of fish that get very large are also widely available in the pet trade in Japan. One very large fish available to aquarists for purchase is the pirarucu of South America. These fish can get enormous, in excess of 2.5 meters (8.2 ft) and over 100 kg (220 lbs). Many people who buy them are not prepared for just how incredibly big these fish can actually get. This could lead to a scenario where we have surprised aquarium owners releasing fish that they are no longer willing or able to take care of.

Perhaps the biggest challenge we face when using introduced exotic species as an explanation for Takitaro sightings is the relative remoteness of Otori-ike and the difficulty of accessing it. The nearest road is 8 km (5 miles) away through rough, mountainous terrain. To reach the lake requires at least a 3 hour, demanding hike, and once there, even more work to get down the steep surroundings to the shore. This seems like it would be an awful lot of trouble for someone to go through just to release a few fish into the lake. With brook trout and rainbow trout, there was a funded, organized program focused on putting a large amount of fish there. However one wonders if a only one or a few people with no such logistic support would go through the trouble and resources of trekking out into the mountains with live fish in order to secretly dump them in Otori-ike.

As with the snakeheads, it is also unlikely that tropical fish such as pirarucu would be able to survive for any appreciable length of time is this cold mountain environment. Many of the larger aquarium species are simply not compatible with this sort of freezing, high altitude habitat, and would likely die in short order.

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Images of various candidates for possible explanations are, from top to bottom:
1) Sakhalin taimen,
2) snakehead (this one is a Northern snakehead),
3) alligator gar,
4) pirarucu, and
5) sturgeon (shown below).

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Another question it seems worth asking is if the Takitaro is perhaps something that has always been there. One avenue of inquiry is that perhaps the fish are a relic population of an ancient, extinct species, or even an unknown fish species that was trapped when the landslide formed Otori-ike from a stream long ago. Indeed Takitaro have been sighted in the lake since the beginning of historic human settlement in the area, which is namely concentrated in nearby Asahi village. If sightings have been consistent for so long, this leads credence to the idea of some large fish species became isolated in the lake during its formation in ancient times.

Pumped up super fish, out of place species, exotics, ancient giant fish, or just plain unknown; whatever they are, Takitaro have achieved a somewhat legendary status in Japan. Indeed even to this day, many a fisherman has thrown their line into the clear waters of Otori-ike wary of any large shadows lurking under the surface and anticipating the possibility that they may receive a mighty tug from this colossus of the depths.